The present invention is directed generally to solid state devices and, more particularly, to such devices having a ZQ pad or terminal which may be used to control the output impedance of the device.
Semiconductor devices such as microcomputers, memories, gate arrays, among others, include input/output pins and an output circuit for transmitting data to other devices, via a bus, transmission lines formed on a board, and the like. A circuit within the semiconductor device that is responsible for transmitting data includes, for example, output buffers and drivers. For there to be optimum transmission, the impedance of the transmitting device should be matched to the impedance of the transmission network and receiving device.
As operational speeds of electronic devices increase, the swing of transmitted signals decreases. However, as the signal swing width of a transmitted signal decreases, external noise increases. External noise can affect the reflection characteristics of an output signal if there is an impedance mismatch at an interface. Impedance mismatches are caused by external noise or by noise on a power supply voltage, temperature and process variations, among others. If an impedance mismatch arises, the transmission speed of the data decreases, and the data from a semiconductor device may become distorted. Thus, in a case where a semiconductor device receives distorted data, problems can be caused by setup/hold failures or errors in reading received data.
U.S. Pat. No. 6,947,336 entitled Semiconductor Device With Impedance Control Circuit discloses a semiconductor device with an impedance control circuit capable of automatically obtaining a desired output impedance value irrespective of power supply voltage, temperature, and process variations. Disclosed therein is a semiconductor device that includes an output buffer circuit having a pull-up section comprised of pull-up transistors and a pull-down section comprised of pull-down transistors; a pad connected with an external resistor; and an output impedance control circuit that is connected to the pad and the output buffer circuit and controls an impedance of the output buffer circuit according to an impedance of an external resistor. A first transistor is connected to the pad. A first current source circuit supplies DC current to the pad, and a first level controller is connected to the pad and controls a gate voltage of the first transistor such that the pad is established at a predetermined voltage. A second transistor is connected to a first internal node and is controlled by the first level controller. A first variable impedance circuit is connected to the first internal node, and a second current source circuit supplies DC current to the first internal node. A first controller, responsive to a voltage variation of the first internal node, generates a first control code for controlling the first variable impedance circuit so that a voltage of the first internal node is established at the predetermined voltage. A first conversion circuit receives the first control code and converts the control code into a string of data bits. The data bits of the string are transferred in series to the output buffer circuit via a single transmission line.
FIG. 1 illustrates a typical prior art impedance control circuit 10. The impedance control circuit 10 has a ZQ pad or a control pad 12 to which an external resistor 14 may be connected. The value of the voltage at the control pad 12 (VZQP) is input to a comparator 16 along with a reference voltage (Vref) produced by a reference generator 18. The comparator 16 is of the type which produces up and down pulses in response to the difference in magnitude between the values of Vref and VZQP. The up and down pulses are filtered by a filter circuit 20 and input to a counter 22. The counter 22 produces a multi-bit, variable count signal 23 representing a count value which is responsive to the number of up and down pulses which have been counted. The count signal 23 is input to a variable impedance circuit 24. The variable impedance circuit 24 is shown in detail in FIG. 2A.
In FIG. 2A, the variable impedance circuit 24 is comprised of four P-channel MOS transistors connected in parallel. The gate of each of the transistors is responsive to one of the bits of the variable count signal 23. Furthermore, each transistor is twice the size, i.e. has twice the drive, of the previous transistor. Thus, transistor P2 is twice the size of transistor P1, transistor P4 is twice the size of transistor P2, and transistor P8 is twice the size of transistor P4.
In FIGS. 3A and 3B, the voltage Vref is compared to the voltage VZQ. At time T0, the voltage VZQ is less than the voltage Vref such that a plurality of up pulses is produced. At time T1, a sufficient number of up pulses has been produced so as to change the value of the variable count signal 23 so as to turn on another transistor within the variable impedance circuit 24. For example, transistor P1 may be turned off and transistor P2 turned on, thereby increasing the value of voltage VZQ by one step. At time T2, the voltage VZQ is still less than the voltage Vref in both cases and, due to the continued counting of the up pulses, the value of the voltage VZQ is increased by the other step, e.g. transistor P1 is turned on while transistor P2 remains on. At time T3, the value of VZQ is increased by another step and now the value of VZQ exceeds the value of Vref. However, in case 1, the amount of overshoot is greater than the amount of overshoot in case 2. In both cases, however, the value of VZQ is recognized as greater than value of voltage Vref so that the counter 22 begins to receive down pulses from the comparator 16 for the period shown in the figure from time T3 to time T4. As a result, the value of the variable count signal 23 is returned to the value of that signal at time T2 such that the voltage VZQ is reduced by one step as shown at time T4. Thereafter, a pattern is developed in which the value of VZQ is increased by a step for one time period, e.g. time T5 to time T6, and is then reduced by one step. By monitoring the pattern, the count of the variable count signal 23 can be locked at either the value which produces the overshoot as shown from time T3 to time T4 or the value which produces a value for the voltage VZQ as seen in the time period T4 to T5. After the value for the variable count signal 23 has been locked, a similar process is carried out for a second variable impedance circuit 26 which is comprised of a plurality of N-channel MOS transistors as shown in FIGS. 1 and 2B.
Returning to FIG. 1, the variable impedance circuit 26 is connected in series with a variable impedance circuit 24′ which is the same as the variable impedance circuit 24. A voltage VZQN available at a node between the first variable impedance circuit 24′ and the second variable impedance circuit 26 is input to a comparator 28. The output of the comparator 28 is filtered in a filter circuit 30 and input to a second counter 32. The output of the second counter 32 varies the impedance of the variable impedance circuit 26 in a manner similar to that discussed above. When the pattern of up and down pulses becomes stable (repeatable), the value of the count of the variable signal produced by the counter 32 is locked.
Typically, the variable impedance circuit 24 comprised of P-channel MOS transistors is calibrated first via an enable signal input to an enable transistor 34. Similarly, the second variable impedance circuit 26 is calibrated after a second enable transistor 36 is rendered conductive. As seen from the foregoing discussion, and particularly with respect to case 1 of FIG. 3A and case 2 of FIG. 3B, the calibration of VZQP may have an error substantially equal to one step. Because the calibration of VZQN is performed after calibration of VZQP, the voltage VZQN may have a two step error. A two step error is a substantial error, particularly for the fast corner.
Accordingly, an improved method and apparatus for ZQ calibration is needed which has high resolution.